It is proposed to apply the most advanced and promising quantitative procedures to the study of platelet and fibrin thromboembolism as well as the hemostatic reactions, in living blood vessels of the microvascular system, and to develop effective means of detecting and controlling these reactions. Investigations will focus on the biophysical and biochemical changes that occur during thromboembolism and hemostasis. Three major approaches will be utilized: in vivo experimentation with emphasis placed on direct microscopic observation and quantitative production and analysis of events; development and application of new instrumentation pertinent to the in vivo efforts; and in vitro experimentation supportive of the in vivo research. The main effort will be on the in vivo approach emphasizing direct optical observation of the events coupled with their quantification. In vivo investigations will include continued study by our existing techniques and instrumentation of the basic mechanisms associated with thrombosis and thrombus susceptibility, vascular fragility, innervation and responsiveness of vascular smooth muscle, platelet-endothelial interactions, and rheological variables. The major new approaches will include (1) development and application of laser doppler heterodyning to measure the velocity profile, shear rate at the vessel wall and along the extent of the thrombus, and the number density ("counts") of platelets and RBC in the measuring volume; (2) study of human and animal blood platelets and red blood cells (RBC) in one or more animal models developed for this purpose; (3) development and application of microspectrophotometry and fluorescence to the study of platelets, RBC, and endothelial cells in vitro and in vivo; and (4) use of ion-selective microelectrodes to measure intracellular, extracellular, and plasma Na ion, K ion and Ca2 ions activities, as well as pH and PO2, and assessment of the role of these variables in thrombosis and hemostasis.